1. Field of the Invention
The present invention relates to an electrochemical biosensor formed by screen printing and a method of fabricating such biosensor.
2. Description of the Related Art
Recently, electrode sensors have been commercially utilized successfully for the fabrication of a variety of clinical measuring products, such as blood sugar, uric acid and cholesterol measuring devices, for their easy and low cost production processes and wide application of cheaper portable measuring devices. Taking the biggest and the most widely used blood sugar measuring device on market as an example, the leading manufacturers include Roche, Abbott, Bayer and Therasense and all of which fabricate blood sugar sensors by electrochemistry. The first generation of such sensors requires higher amount of blood sample (5–10 μl and above) and takes longer (30–60 seconds) to measure a sample. Hence, they are still not considered ideal although the amounts of blood sample and measuring time they require are much less than those conventional colorimetric method does. As technology has improved over the years, the latest generation of sensors only requires 0.3 μl (Freestyle by Therasense) or 1 μl (OneTouch Ultra by Lifescan), and measuring time has also been reduced to 5–10 seconds. Such sensors have become a guide for products of a similar kind and technological development, as well as for further research and development of different electrode structures.
U.S. Pat. No. 5,437,999 by Diebold et al in 1995 has disclosed a sensor including opposing working and counter electrode elements spatially displaced by a spacer having a cut-out portion forming a capillary space between the working and counter electrode elements and a vent port in the working or counter electrode where air can be vented. A precise minute amount of a sample can be introduced via the capillary space and brought into contact with electrodes and reagents. Such sensors can be fabricated by photolithography or screen printing but processes of affixing two insulating substrates with an electrode thereon are very complicated and expensive. U.S. Pat. No. 5,779,867 by Shieh in 1998 has also disclosed a glucose sensor generally comprising a sensor electrode, a reference electrode, and a corpuscle separation thin film carrier strip sandwiched therebetween, which can filter erythrocyte, and an opening where a sample can be introduced. The carrier strip can be used to control volume of the sample flowing into the carrier strip and to remove interruption of erythrocyte during reactions. However, the amount of the sample introduced and the speed of filtering cannot be effectively and efficiently controlled. U.S. Pat. No. 6,129,823 by Abbott has proposed an electrode strip in which electrodes are covered with one or more mesh layers. The improvement involves a partial occlusion of the mesh which underlays an aperture within an upper cover above the mesh, and the aperture is formed above or adjacent to a working electrode. The partial occlusion can reduce the total volume of blood required to perform a measurement. Such sensor only requires 2.0–2.5 μl of the sample but applies a mesh to reduce the volume of blood and distribute the sample. U.S. Pat. Nos. 6,299,757 and 6,338,790 by Therasense have also suggested two opposing working and counter electrodes with a highly hydrophilic thin film finely constructed therebetween, which can introduce a sample to a sample chamber. The volume of the sample can be strictly controlled down to 0.3 μl by the water hydrophilic thin film, which is the lowest in the field. However, the processes of fabricating such sensors are very complex and extremely costly. ROC (Taiwan) Patent Publication No. 268,095 by Shieh has disclosed the technique of electrode fabrication by screen printing, in which an electrically conductive film and insulating layer are produced by screen printing. A metal layer is formed by electroplating and a circular recess, containing a so-called bio layer, is formed by coating a working and a reference electrodes with insulating paste. Sample of about 10 μl can be dropped to the recess to be measured. Such technique requires a larger amount of sample and processing such sensors introduces numerous electroplating process steps. ROC (Taiwan) Patent No. 124,332 by Apex Biotechnology Corp. has disclosed an inflow area formed above an electrode area. Mesh containing surfactant is spread above the inflow and electrode areas and sample can be brought into the electrode area by capillary or siphon. Such application is similar to that developed by Abbott, which utilizes mesh for the inflow of sample and is thus more costly, is also restricted to the amount of sample required.
U.S. Pat. No. 6,258,229 by Winarta et al in 2001 has disclosed a disposable electrode strip, which claims to require less than 1 μl of liquid sample. A piece of gold/polyester or tin oxide/gold polyester film is cut to shape, forming a base layer of sensor. A CO2 laser is used to score the gold or tin oxide/gold polyester film and the film is scored by the laser creating scoring line such that two electrodes at sample end and three contact points are formed at an electrical contact end. A piece of double-sided tape is cut to size and shape, forming middle layer with a U-shaped channel, which contains an electrode area. A top layer, which is placed and coextensive with the middle layer, has a vent opening, which forms a fluid sample channel between sample inlet and the middle of the vent opening, which enables the fluid channel to restrict the volume of fluid to less than 1 μl. Such design is similar to that disclosed in U.S. Pat. No. 5,120,420 by Nankai et al in 1992, except that electrodes are formed in a different way. The electrode sensor disclosed by Nankai et al is a bi-electrode sensor by screen printing an insulating board. A fluid channel is formed by transversely adhering two spacers on opposing ends of electrodes and a top layer without an opening on top of spacers, which in turn forms a channel transverse to a working electrode. By this way, the volume of sample flowing into the channel cannot be controlled and the sample is likely to float a vent opening, which causes contamination. Another improvement employed by Winarta et al, which applies a middle layer with a U-shaped opening on top of a working electrode and subsequently a top layer with a vent opening over the middle layer, forms a fluid sample channel between sample inlet and the middle of the vent opening. With this structure, sample fluid may float the vent opening when the size of which is too small. On the other hand, sample fluid will be retained at the edge of the vent opening when the size of which is appropriate. However, as the size of sensors is getting smaller, it is likely to touch the vent opening by hand which causes outflow of sample fluid and thus contamination.
From the above analysis, it is understood that in order to achieve smaller volume of sample fluid and faster analysis yet avoid any possible contamination, it is necessary to design electrodes which incorporate capillary and siphon.